The present invention relates to fuel cells, and more particularly to fuel processors for fuel cells.
Fuel cells are increasingly being used as a power source in a wide variety of different applications. Fuel cells have been proposed for use in electrical vehicle power plants to replace internal combustion engines. A solid-polymer-electrolyte membrane (PEM) fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H2) is supplied to the anode and air or oxygen (O2) is supplied to the cathode.
In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (exe2x88x92). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane while the electrons flow through an electrical load that is connected across the electrodes. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+) and electrons (exe2x88x92) are taken up to form water (H2O).
The main function of a fuel processor in the fuel cell system is to provide a controlled hydrogen-rich stream to the fuel cell stack. The fuel cell stack converts the chemical energy in the hydrogen to electrical power to charge capacitors or batteries or to directly power a device such as an electric motor. In hybrid applications, a storage medium such as capacitors or batteries removes some of the problems that are associated with transient demand. For non-hybrid applications, the fuel processors need to provide a highly dynamic flow rate of hydrogen-rich gas to the fuel cell stack. When a device is directly powered by the fuel cell, the amount of hydrogen that is required is determined by the demand for power output from the fuel cell. For example in automotive applications, the driver demands power by depressing the accelerator pedal. Acceleration requires the electric motor to turn faster, which requires more current. When the accelerator is depressed, the fuel processor increases the hydrogen that is provided to the fuel cell. The current output by the fuel cell increases and the electric motor accelerates the vehicle.
The fuel processor produces a reformate stream that is composed primarily of hydrogen, carbon dioxide, nitrogen, water, methane and trace amounts of carbon monoxide. During operation, the fuel processor determines the flow rate of hydrogen that is required to meet the current demand for power. As can be appreciated, the demand for power can vary significantly. For example, a vehicle moving in rush hour traffic may repeatedly require sudden acceleration followed by deceleration or braking. Thus, the delivery of hydrogen to the fuel cell stack must vary accordingly. Fuel processors also require careful metering of air and fuel to maintain precise oxygen-carbon ratios in the reformate stream.
While the fuel cell stack can consume as much hydrogen as it needs based on the electrical load applied to the fuel cell stack, mismatching the hydrogen flow and the electrical load is problematic. An under-fueled stack may cause some of the fuel cells to temporarily have reverse polarity, which may damage the fuel cell stack. An over-fueled stack will not damage the fuel cell stack but may increase the combustion temperature. Increased combustion temperature may damage the combustor or cause NOx emissions to increase.
A fuel processor control system for a fuel cell according to the invention includes a fuel cell stack and a fuel processor. A water metering device controls water provided to the fuel processor. A fuel metering device controls fuel provided to the fuel processor. An air flowrate sensor generates an air flowrate signal based on air flowing to the fuel processor. A valve is located between an outlet of the fuel processor and an inlet of the fuel cell stack. A controller is connected to the water and fuel metering devices, the air flowrate sensor, and the valve. The controller controls the valve and the water and fuel metering devices based on the air flowrate signal.
In other features of the invention, the air flowrate sensor is replaced by other feedback signals. In one alternate embodiment, the fuel cell stack generates a stack voltage signal or a stack cell voltage variation signal. The controller controls the valve and the water and fuel metering devices based on the stack voltage signal or the stack cell voltage variation signal. In another alternate embodiment, a pressure differential sensor is connected to an inlet and an outlet of the valve and generates a pressure differential signal. The controller controls the valve and the water and fuel metering devices based on the pressure differential signal. In another embodiment, a flowrate sensor is connected between the valve and the fuel cell stack. The flowrate sensor provides a stack flowrate signal. The controller controls the valve and the water and fuel metering devices based on the stack flowrate signal.
In still other features of the invention, the fuel processor includes a reformer and a watergas shift reactor that is located between the reformer and the valve. The reformer is preferably an auto thermal reformer or a partial oxidation reformer.
In yet another feature of the invention, the fuel processor includes a steam reforming reactor.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.